CRON often returns ambiguous values

Description

Many of the cron API functions return ambiguous values. For example, wp_cron()​returnsnull (explicitly) when cron doesn't run, but it also returns null (implicitly) every other time. This can make debugging harder.

I think this is good. While there are technically return value changes, I'm not too concerned about the back compat.

wp_clear_scheduled_hook() probably just needs to return true (it got cleared) or false, versus an array. It's also possible we change how that function works in the future, #25773 gave us some trouble.

The wp_remote_post() call is non-blocking, so spawn_cron() should probably not have a return value, right? Thus wp_cron() shouldn't have a return value either. I know that was the original bug report — what about true? Is that sufficient?